Conventionally used metal substrates for outer panels for consumer electronics, materials for furniture, members for building interiors, etc., are laminates obtained by applying vinyl chloride resins (hereinafter also referred to simply as “vinyl chloride”) to the surface of such metal substrates, followed by lamination. However, environmental problems have recently been highlighted, and polyolefin resins have been proposed as an alternative to vinyl chloride. Polyolefin resins are widely used in various fields because they are non-toxic, highly resistant to acid, alkali, organic solvents, etc., excellent in mechanical strength and abrasion resistance, and inexpensive.
However, polyolefin resins, which were non-polar, had difficulty in bonding with metal substrates. Various adhesives have been proposed to bond such polyolefin resins and metal substrates. Typical adhesives are those obtained by mixing a solvent-dispersed modified polyolefin resin in a base adhesive in which a thermosetting resin (e.g., phenolic resin, melamine resin, or epoxy resin) or a thermoplastic resin is dissolved in an organic solvent. However, these adhesives hardly had sufficient adhesion. Furthermore, PTL 1 proposes an adhesive composition obtained by mixing an acid-modified polyolefin and an epoxy resin with catechol etc., and PTL 2 proposes an adhesive composition obtained by mixing a maleic acid-modified chlorinated polypropylene, a chelate epoxy resin, and a solvent.